Qwerty Row Sums

#!/usr/bin/env python3
import sys, argparse

argparser = argparse.ArgumentParser()
argparser.add_argument('--wordfile', default='/usr/share/dict/words')
argparser.add_argument('--target', default=64, type=int)
args = argparser.parse_args()

qwerty = ['QWERTYUIOP', 'ASDFGHJKL', 'ZXCVBNM']
letters = {}
for row in qwerty:
    for i, letter in enumerate(row):
        letters[letter] = i + 1
        letters[letter.lower()] = i + 1

try:
    with open(args.wordfile, 'r') as wordlist:
        for word in wordlist:
            word = word.strip()
            if sum(letters[letter] for letter in word if letter in letters) == args.target:
                print(word)
except IOError:
    print('Error: Unable to read word list \'{0}\''.format(args.wordfile), file=sys.stderr)